In a typical cellular radio system, wireless user equipment units (UEs) communicate via a radio access network (RAN) to one or more core networks. The user equipment units (UEs) can be mobile telephones laptop computers with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network. Alternatively, the wireless user equipment units can be fixed wireless devices, e.g., fixed cellular devices/terminals which are part of a wireless local loop or the like. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a radio base station (e.g., BTS, RBS or NodeB). The radio base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UE) within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a control node known as a base station controller (BSC) or radio network controller (RNC). The control node supervises and coordinates various activities of the plural radio base stations connected thereto. The radio network controllers are typically connected to one or more core networks. One example of a radio access network is the Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN). The UMTS is a third generation system which builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. UTRAN provides wideband code division multiple access (WCDMA) to UEs.
In many radio access networks the radio base station is located concentrated at a single site. However, a radio base station can also have a distributed architecture. For example, a distributed radio base station (RBS) can take the form of one or more radio equipment (RE) portions that are linked to a radio equipment control (REC) portion over a radio base station internal interface.
Such distributed radio base stations can have a processing Main Unit (MU) at the REC, and at the RE a set of antennas with dedicated RF equipment able to cover multiple radio cells (RRUs), where a single MU is shared among multiple RRUs. This new architectural approach in the RBS implementation requires high capacity, cost effective and low latency transport systems between MU (processing) and RRUs (antennas).
The Common Public Radio Interface (CPRI) is an example of an internal interface of a radio base station which links a radio equipment portion of the radio base station to a radio equipment control portion of the base station. The Common Public Radio Interface (CPRI) is described in Common Public Radio Interface (CPRI) Interface Specification Version 5.0 (2011). Other interfaces can be used, for example the Open Base Station Architecture Initiative (OBSAI) but such alternatives have not yet proved as popular.
This approach of providing “remotization” of the RF part of the RBS from the main unit can bring some notable advantages:                a) Rationalization of RBS processing unit, with benefits in terms of cost and power consumption,        b) Dynamic allocation of RF and/or processing resources depending on cell load and traffic profiles, and        c) Correlation of data supported by all the antennas which are afferent on the same processing unit. It increases radio link reliability, bandwidth, and coverage and optimizes the power consumption.        
This can enable some “cloud computing” concepts to be applied to the radio access networks. Point to point (P2P) optical links can be used for the interface between the REC and the RE. For this interface, WDM systems, especially the ones used in the access (WDM-PON), can enable guaranteed low latency, protocol transparency, high bandwidth and an increased spectral efficiency. The costs, over a 2-5 year time scale projection, can be comparable with conventional optical access technologies, such as P2P and GPON. CPRI can be transmitted over P2P dedicated optical links. Notably CPRI has pressing constraints in terms of latency (round-trip delay) and in particular in terms of uplink/downlink synchronization.
The CPRI standard recites optical fibers for transmission link up to 10 km, recites determining a round trip delay, and specifies synchronisation and timing accuracies, e.g. link round trip delay accuracy of 16 nsecs.